The invention relates to a vehicle wheel suspension having a hydraulic vibration damper, which is connected in parallel to a suspension spring that proportionally supports the vehicle body, and in which vibration damper a rebound spring is provided, which preferably and typically first becomes effective in the event of larger rebound travel. The support point of the rebound spring is displaceable in relation to the damper housing by a spring support piston, which is provided therein and is displaceable from the outside by a fluid supplied with pressure via a pump. With respect to the prior art, reference is made to DE 10 2004 019 991 A1, as well as DE 101 21 918 B4.
In the above first-mentioned DE 10 2004 019 991 A1 document, a vibration damper of a wheel suspension having an auxiliary spring made of an elastomeric material is described, which is connected in parallel to the typical suspension spring and the support of which in relation to the vehicle body is adjustable via an activatable piston arrangement in relation to the longitudinal axis of the piston rod of the damper, which is referred to hereafter in the present case as the damper rod. This adjustment is performed hydraulically with delivery of the hydraulic medium using a pump driven by a motor, which can be assisted in particular by a hydraulic pressure accumulator, to display a high system dynamic response. A level-regulating intervention is possible using such an adjustment of the auxiliary spring, i.e., the vehicle body is to be able to be raised or lowered as needed.
A spring-damper system having a springy stop is also described in the second-mentioned DE 101 21 918 B4, wherein both an auxiliary spring, which acts as a pressure stage stop and is formed by an elastomeric body, and also a rebound spring for the traction stage of the damper, can be influenced by way of so-called stop distance in such a manner, or are concretely displaceable within the damper housing along the damper rod, such that the tendency of the vehicle body to roll in relation to the wheels when cornering is counteracted.
In the present case, it has been recognized that in any case when a suspension spring, which is in parallel to the damper, is implemented in the form of a typical mechanical coiled spring, solely a displacement ability of the rebound spring, as disclosed in the above-explained document, which comes into effect in the event of stronger spring deflection of the vehicle body in relation to the wheels, i.e., in the event of a larger body movement away from the roadway, opens up the possibility of intentionally lowering the vehicle body, without, as specified in DE 101 21 918 B4, a change of the suspension spring, which is implemented in this document as an air spring, having to be performed for this purpose.
It is hereby to be disclosed for such a vehicle wheel suspension, how the energy demand of the pump required to change the height level of the vehicle body above the roadway can be kept low or how, if the minimization of the energy demand is not a paramount goal, such a wheel suspension can be designed particularly safely with respect to the parking of the vehicle. The solution to this object is characterized in that the pump is connected to a pressure accumulator for the fluid and fluid is delivered out of this pressure accumulator into the damper housing or out of the damper housing back into the pressure accumulator, wherein the absolute value of the spring constant of the pressure accumulator is at least in the order of magnitude of the spring constant of the associated suspension spring or, in the case of multiple suspension springs having associated vibration dampers, which are associated with one pressure accumulator, of the spring constants of these suspension springs, or greater.
If a pressure accumulator is provided for the fluid, which is delivered with overpressure by a pump for a desired displacement of the spring support piston in the damper housing, from which pressure accumulator this pump continuously already acquires the fluid under overpressure, of course, less energy is thus required for this fluid delivery than if the pump had to suction the fluid from an “unpressurized” collection container. Energy is also required for the pressure buildup in the pressure accumulator, however, in the event of reasonable activation, a part of the power, which reaches the rebound spring in the event of a spring deflection movement of the vehicle body and is quasi-released in this case, can be used for this purpose. Furthermore, the time factor is a significant advantage of such a pressure accumulator, in that with the aid of its stored energy, a desired displacement of the spring support piston can be displayed within a relatively short time span, while the renewed pressure buildup in the pressure accumulator can be performed extended over time, so that the required delivery power of the pump can be kept relatively low.
According to the invention, the absolute value of the spring constant of the pressure accumulator is at least in the order of magnitude of the spring constant of the associated suspension spring, or greater. A pressure accumulator for a fluid always has a spring element in the broadest meaning, which is tensioned further upon charging of the pressure accumulator with a pressurized fluid. For example, this spring element can be a coiled compression spring element, which is supported on a displaceable piston, which delimits the pressure chamber of the pressure accumulator. Alternatively, a gas spring volume is also possible. Of course, such a spring element of a pressure accumulator has a defined spring constant (also called spring rate or spring stiffness). If this spring constant has the same absolute value as the spring rate of the suspension spring assigned to this pressure accumulator, via which the vehicle body is proportionally supported on the respective wheel of the vehicle, theoretically, in any case, no pump power is required for the displacement of the spring support piston, which is provided in the respective vibration damper, in the sense of tensioning the rebound spring provided therein. However, friction losses are actually to be overcome, for which a small pump power is required. Of course, however, it is then relatively small.
In the explanation of the preceding paragraph, it was presumed that a single vibration damper having a spring support piston, with which a single suspension spring is associated, is associated with a pressure accumulator according to the invention, i.e., this explanation relates to the spring-damper unit in the wheel suspension of a single vehicle wheel. However, this also applies for an arrangement which is optimized with respect to the construction expenditure, in which a single pressure accumulator according to the invention is associated with all wheels of the vehicle (in the case of a passenger automobile, these are the two front wheels and the two rear wheels). Four suspension springs are then active in a parallel arrangement to one another. However, simultaneously four vibration dampers according to the invention, each having one spring support piston, are also provided, which are also active in parallel to one another. Thus, if the spring constants of all suspension springs are equal, such a parallel circuit of the suspension springs to the respective associated vibration dampers thus remains without an effect with respect to a single associated pressure accumulator, i.e., the relationships explained in the preceding paragraph also apply in unchanged form for this purpose. For this reason, it is discussed, that in the case of multiple suspension springs having associated vibration dampers, which are associated with one pressure accumulator, the absolute value of the spring constant of the pressure accumulator is at least in the order of magnitude of the spring constants of the suspension springs or greater. These explained relationships also apply in this case if the spring constants of the multiple suspension springs only differ slightly from one another. However, if significantly harder (or softer) suspension springs (i.e., those having substantially higher or substantially lower spring constant) are installed on one axle of the vehicle than on the other axle of the vehicle, it could be advisable to provide a separate pressure accumulator according to the invention for each of these axles of the vehicle.
Heretofore, only the advantage was explained which results from a substantially equal absolute value of the spring constant of the pressure accumulator and the spring constant of the associated suspension spring(s). If the absolute value of the spring constant of the pressure accumulator is greater than the spring constant of the suspension spring(s) associated with this pressure accumulator, a safety function is provided, with the aid of which, in the event of suitable switching of a valve, which is provided in the hydraulic (or fluidic in general) connection between the pressure accumulator and a chamber (so-called support chamber), which is provided in the damper housing and is delimited by the spring support piston, it can be ensured that the vehicle body moves into its completely lowered position when the vehicle is shut down and also maintains this position until the vehicle is put back into operation. This can be necessary, for example, in low garages or on decks of ferries having a low ceiling height. This mentioned valve must only be opened and remain open and therefore allow a fluidic connection between the pressure accumulator and said support chamber in the damper housing, which can be provided easily and without energy demand using a solenoid valve which is open when deenergized (or an arbitrary bistable valve).
In the sense of an advantageous refinement, the mentioned spring support piston, which, while displaceable in the damper housing, delimits a support chamber fillable with the fluid, is held by a spring element, which is preferably supported on the rebound spring, in such a starting position that the volume of the support chamber, which essentially has ambient pressure, is minimized. With an intentional displacement of this spring support piston and a buildup of over pressure required for this purpose in the support chamber, the force of this mentioned spring element must then be overcome, however, the energy demand required for this purpose can be kept relatively small, if the spring force of this spring element is selected to be as small as possible.
Furthermore, it is proposed, as an operating method for a wheel suspension according to the invention, that a support chamber in the damper housing, which is fillable with the fluid (as which, in addition to a hydraulic medium, a gaseous medium and preferably air can also expressly be used), and via which the spring support piston is supported, is always to be set substantially free of overpressure by delivering fluid back to the pressure accumulator by use of the pump, when the height level of the vehicle body is not to be changed by targeted pre-tension of the rebound spring. Therefore, friction losses in the vibration damper are kept as small as possible. However, it is to be ensured that no overpressure is generated in the support chamber, in particular to avoid an undesired penetration or suctioning of air via piston seals or the like, in particular if a hydraulic medium is used as the fluid. In particular, a corresponding valve can be provided for this purpose in the line from the support chamber to the suction side of the pump, which valve blocks this line upon the presence of ambient pressure in the support chamber and simultaneous suction operation of the pump. Alternatively thereto, a so-called compensation volume can be provided, which can also prevent ambient air from being suctioned into the damper during emptying of the support chamber.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.